A lysosome is a cellular organelle. Various materials (foreign materials, waste materials, etc.) in a cell are broken down by action of lysosomal enzymes present in a lysosome. It is known that there are more than 50 kinds of lysosomal enzymes. Lysosomal enzymes optimally work at a pH in an acidic environment. A deficiency of a lysosomal enzyme due to a genetic defect causes accumulation of a substrate in a cell, which leads to a lysosomal disease. At present, more than 40 kinds of such congenital lysosomal diseases are known. However, a fundamental therapeutic method has not been found yet.
Lysosomal diseases have been treated by making up for the deficient enzymes, and for this purpose, enzyme replacement therapy, bone marrow transplantation, and gene therapy have been attempted. To date, lots of lysosomal enzymes have been identified and linked with lysosomal diseases.
For enzyme replacement therapy, a recombinant enzyme has been developed using a cell line strongly expressing one particular lysosomal enzyme. To date, 7 kinds of enzyme preparations comprising such a single recombinant lysosomal enzyme have been marketed, i.e., a Gaucher's disease therapeutic agent, CEREZYME (registered trade mark) (a recombinant imiglucerase preparation); a Fabry disease therapeutic agent, FABRAZYME (registered trade mark) (a recombinant agalsidase beta preparation); a Fabry disease therapeutic agent, REPLAGAL (registered trade mark) (a recombinant agalsidase alfa preparation); a mucopolysaccharidosis type I therapeutic agent, ALDURAZYME (registered trade mark) (a recombinant laronidase preparation); a mucopolysaccharidosis type II therapeutic agent, MYOZYME (registered trade mark) (a recombinant alglucosidase alfa preparation); a mucopolysaccharidosis type II therapeutic agent, ELAPRASE (registered trade mark) (a recombinant idursulfase preparation); and a mucopolysaccharidosis type VI therapeutic agent, NAGLAZYME (registered trade mark) (a recombinant galsulfase preparation).
However, these therapeutic agents are directed to treat lysosomal diseases caused by a deficiency of a single lysosomal enzyme, and are not effective against lysosomal diseases caused by deficiencies of two or more lysosomal enzymes. In addition, the above-mentioned therapeutic agents which are currently marketed are very expensive. Therefore, there is a need for development of inexpensive therapeutic agents.
It is known that a mannose 6-phosphate residue is added to lysosomal enzymes when they are synthesized within a cell, and thereby they are transported via a mannose 6-phosphate receptor to a lysosome.
Based on this fact, for the purpose of effective delivery of an enzyme to a lysosome in an enzyme replacement therapy, some studies to obtain highly-phosphorylated lysosomal enzymes have been conducted. For example, a method of modifying an isolated lysosomal enzyme with a mannose 6-phosphate residue (see Patent Literature 1), and a method of preparing a highly-phosphorylated recombinant lysosomal enzyme using a special cell line (see Patent Literature 2) are reported. However, using these techniques, it is difficult to produce many kinds of lysosomal enzymes at once and inexpensively.
Examples of lysosomal diseases caused by deficiencies of two or more lysosomal enzymes include mucolipidosis type II (hereinafter, referred to as “ML-II”), and mucolipidosis type III (hereinafter, referred to as “ML-III”) which is a mild type of ML-II. These lysosomal diseases are caused by a defect in GlcNAc-phosphotransferase, which adds a mannose 6-phosohate residue to lysosomal enzymes synthesized in a cell. In ML-II and ML-III, a mannose 6-phosohate residue is not added to lysosomal enzymes synthesized in the cell due to a defect in GlcNAc-phosphotransferase, and therefore the lysosomal enzymes are not recognized by a mannose 6-phosphate receptor and then are not transported to lysosomes. Thus, patients with ML-II and ML-III have deficiencies of almost all of lysosomal enzymes in lysosomes.
Therefore, administration of a single enzyme preparation as mentioned above is not effective for the treatment of ML-II and ML-III. For the treatment of ML-II and ML-III, it is theoretically necessary to replace all of the deficient lysosomal enzymes. However, it is very difficult to isolate and purify all of more than 50 kinds of lysosomal enzymes individually in which ML-II and ML-III patients are deficient. In addition, recombinant lysosomal enzyme preparations as mentioned above are very expensive. It is also economically difficult to use several tens of the expensive enzyme preparations.
Under such circumstances, conventional enzyme preparations using single recombinant lysosomal enzymes cannot be applied to the treatment of such lysosomal diseases as ML-II and ML-III. Therefore, new useful therapeutic agents for lysosomal diseases are desired.